fix: install libev before gevent to fix macOS harness build

gevent requires libev to compile its C extensions, which is not
pre-installed on macos-latest runners.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: robert-hebel-sb

.github/workflows/build.yml

@@ -38,6 +38,9 @@ jobs:
       with:
         node-version-file: .nvmrc
         cache: 'yarn'
+    - name: Install gevent build dependencies
+      run: brew install libev
+      if: env.install_manually
     - name: Set up Python virtual environment
       run: |
         python3 -m venv venv